To simplify #(-15x^4)/(3x)#, we must divide #-15x^4# by #3x#. -15 divided by 3 is simple; the answer is -5. I think you might have been strung up on dividing #x^4# by #x#. Here is a Quotient property that will help us out: #(a^n)/(a^m) = a^(n-m)#. Using this, we can figure out that #x^4# divided by #x# is #x^3# since #x^(4-1) = x^3#. In conclusion, the answer is #-5x^3#.